Micro displacement not rendering with motion blur. (Blender 2.79 )
As of 2.79, Cycles does not support adaptive (micro) subdivision and deformation blur on the same mesh. You should uncheck "adaptive" on the subdivision surface modifier if you need deformation blur.

See this documentation page from the main developer of this feature: https://wiki.blender.org/index.php/User:Maiself/Micropolygon_Displacement